This invention relates to stators of submerged motors and methods of producing same, and, more particularly, to a stator of a submerged and a method of producing same which motor may be used with a water circulating pump or other motor which is capable of operation in the water with high reliability.
Generally, a submerged motor is an induction electric motor in which a rotor substantially producing no voltage is placed in such a manner that its coil conductors are in direct contact with the water. However, a stator coil having a high voltage applied thereto should have a watertight construction because an insulation layer of the coil would have its dielectric strength greatly reduced if it were brought into direct contact with the water. Thus, in motors of medium and small sizes of an output power of below 50 KW, it has been usual practice to adopt what is generally referred to as a canned system in which the stator is contained in its entirety in a container of stainless steel by filling the gap with a mold resin incorporating inorganic material therein.
As the submerged motors grow in size and output power, it is necessary to increase the wall thickness of a can of stainless steel to meet the requirement of increasing mechanical strength set by an attendant increase in the inner diameter and length of the stator.
On an inner side of the container or can, an eddy current loss would be produced by a revolving magnetic field generated in a gap between the stator and the rotor. The eddy current loss would be proportional to the thickness of the can and the length of a stator core, and would be proportional to the third power of the inner diameter of the can. Of course, it would be proportional to the electric conductivity of the can. Thus, in submerged motors of large size, the thickness of the can and the length of the stator core would become great, resulting in the eddy current loss reaching several percent of the output power. Consequently, such motor could not be accepted for practical use.
A very effective method of obviating the aforesaid problem would consist in using a can of a composite structure provided by using, as material for the inner side of the stator in which an eddy current loss is produced, a material which is high in water shielding effect and low in eddy current loss (or a material of low electric conductivity). A carbon fiber reinforced plastic (hereinafter CFRP for short) would be one example of such materials which meet the requirement. Since a CFRP has an electric conductivity which is about 1/100 that of stainless steel, it would be possible to reduce the value of an eddy current loss to a level below 1% that of stainless steel if a sheet of a CFRP of a thickness of 1-1.5 mm were used by taking mechanical strength into consideration. Such low eddy current loss would pose no problem in putting the canned submerged motor into practical use.
A stator of a submerged motor of the composite can system of a composite structure using an inner cylinder of CFRP generally comprises a stator core usually mounted in the form of a lamination inside an outer cylinder, and a stator coil contained in a slot formed in the stator core. The inner cylinder of CFRP is fitted to the inner side of the stator core, and end covers are attached to opposite ends thereof through inside joints while being joined to the outer cylinder through outside joints to provide a can. The outer cylinder and the end covers are usually formed of stainless steel of higher electric resistance than other metals. An adhesive agent of the epoxy resin base is used for the inside joints for joining CFRP and stainless steel together by taking watertightness into consideration. The outside joints may be provided by welding. By filling the gap in the can with a mold resin by pouring same through an inlet port formed in one of the end covers or the outer cylinder and allowing the poured mold resin to set, it is possible to obtain a stator as a finished article.
The stator of the submerged motor of the aforesaid construction raises, in putting same to production, a problem which is difficult to solve, on account of the thickness of the inner cylinder of CFRP being very small (1-1.5 mm). More specifically, to obtain a stator of a submerged motor of high quality, it is necessary that the cylinder of CFRP be very high in dimensional accuracy and precision (length, inner and outer diameters, circularity, etc.). However, this requirement is hard to satisfy in articles of small wall thickness.
Generally, in producing a cylinder of CFRP, carbon fibers are wound on a mandrel of metal and impregnated with a resin which is allowed to harden, and then the mandrel is withdrawn from a cylindrical object formed thereon. When the cylindrical object is small in thickness, irregularities might be caused to occur in the carbon fibers constituting the cylindrical object or strain might be applied to the hardened resin as the mandrel is withdrawn. Also, withdrawing of the mandrel might cause a deformation to occur in the cylindrical object by mechanical force used for withdrawing. In actual practice, the cylinder of CFRP of a submerged motor of large size has a diameter in the range between 400 and 500 mm. In particles of this large size, it is almost impossible to reduce the deformation to a level below 1 mm. If deformation occurred, the area of adhesion between the end covers and the cylinder of CFRP would be reduced, and moreover a satisfactory bond could not be obtained between them because of gaps existing between them. To improve the bond strength of the inside joints, it is necessary that the angle of connection between each end cover and the inner cylinder be reduced to increase the area of adhesion between them. In the case of articles of small thickness, if the angle of connection were reduced, a reduction in thickness caused by the reduction in the angle of connection might result in breaking or stripping of the end portions when machining is performed, thereby aggravating the deformation described hereinabove. Thus, in stators of submerged motors of the composite can type of the prior art described hereinabove, a problem has been raised with regard to obtaining good joints between the end covers and the inner cylinder of the can.